1. Field of the Invention
The present invention relates to a testing apparatus for testing electrical characteristics of, for example, a semiconductor wafer, and a connection method for the testing apparatus.
2. Description of the Related Art
In manufacture of semiconductor devices, a number of semiconductor devices are formed on a semiconductor wafer, and the devices are severed from each other. In such a semiconductor device fabricating process, a probe apparatus has conventionally been used and electrical characteristics of semi-finished semiconductor devices, which are still in the form of the semiconductor wafer, are tested.
The probe apparatus has a number of probes which can be put in contact with electrode pads of each chip on the semiconductor wafer. When the probes are put in contact with the pads of each chip, various measurement patterns supplied from a tester are delivered to each chip. An output pattern from each chip is monitored by the probe apparatus. As a result, the electrical characteristics of each chip on the semiconductor wafer are tested.
The probe apparatus has, for example, a structure wherein a test head including a test circuit, etc. is movable between a test position and a retreat position to which the test head is retreated from the test position. In the test position the test head is put in contact with the contact points of a probe card. When the test head is moved to the retreat position, the test head is put out of contact with the contact points. While the test head is in the retreat position, for example, exchange of the probe card is performed in the testing apparatus.
A connection unit is provided between the test head and the probe card for connecting the test head and the probe card. An upper portion of the connection unit has a plurality of terminals to be connected to the wires of the test head, and a lower portion of the connection unit is provided with a plurality of pogo pins functioning as extendible contact elements connected to the terminals. When the pogo pins are put in pressure contact with the electrode portions of the probe card, the pogo pins retreat and by virtue of urging force of springs housed within the connection unit the pogo pins are put in contact with the electrode portions under proper pressure. The connection unit is attached to the test head. When the test head is situated in the test position, the connection unit is fixed to the head plate. A probe card holder which holds the probe card is attached to the head plate.
In the conventional structure, when the test head is swung and connected to the probe card, a virtual plane defined by tip portions of the probes supported by the probe card is not parallel to the wafer to be tested. Consequently, the probes may not come into good contact with the semiconductor wafer.
Specifically, when the test head is swung to the connection position for connection with the probe card, a pressure acts on the probe card holder and a bending moment occurs in the head plate. In particular, when the number of pogo pins functioning as contact elements is great, a considerably great contact load is needed to bring all pogo pins into uniform contact with the electrode portions of the probe card. For example, if a contact load for one pogo pin of the connection unit is 100 g and the number of pogo pins is 1000, a load of about 100 kg acts on the head plate. As a result, the head plate is deformed by the bending moment and consequently the probe card may be deformed. The degree of bending deformation of the head plate in this case may reach 100 to 250 .mu.m or thereabout, although it depends on the strength of the head plate.
If the head plate is deformed, the conditions for contact between the semiconductor wafer and the probe, which are set before the test head is brought to the connection position, e.g. the contact angle and contact position, will vary. If the contact angle varies, the tip portions of the probes will be inclined and the optimal contact in a direction perpendicular to the electrode pads will not be effected. In addition to the variation in contact angle of the probes, displacement of the probes and breakage of the probes may occur. More specifically, if the head plate is deformed, the distance between the probes and the object to be tested varies. In a region where the distance between the probe and the test object is short, the probe may be buckled and broken, or the position of the probe tip may be displaced.
There is another problem in the conventional apparatus in that proper contact between the probe card and the pogo pins is not easily attained, for the following reason.
A support shaft, around which the test head rotates, is normally situated on the upper portion of the head plate. One reason for this is that a passage for automatically exchanging the probe card is defined within the head plate and interference with peripheral devices must be prevented. Besides, since reduction in measuring is demanded in tester specifications, the contact position between the probe card and pogo pins is set at a place near the wafer table within the probe apparatus. Accordingly, a great distance is present between the axis of the support shaft and the contact plane between the pogo pins and the probe card. Thus, when the test head is swung, pogo pins situated near the support shaft come into contact with electrode portions of the probe card earlier than pogo pins situated away from the support shaft. As a result, the pogo pins near the support shaft slide on the electrodes of the probe card until the pogo pins away from the support shaft come into contact with the electrodes of the probe card, and displacement of contact positions may occur in this region where the pogo pins near the support shaft are situated or a contact pressure increases abnormally. Consequently, pogo pins may be broken. This undesirable phenomenon becomes conspicuous as the distance between the axis of the support shaft and the contact plane between the pogo pins and probe card increases.